As fibers having high strength and high heat resistance, a polybenzazole fibers of polybenzoxazole or polybenzothiazole has been known well and fiber formation of the polymers is described in, for example, Patent Documents 1 and 2.    Patent Document 1: U.S. Pat. No. 5,296,185    Patent Document 2: U.S. Pat. No. 5,385,702
Since a polybenzazole fiber and a pyridobisimidazole fiber have capabilities at the highest level in all aspects such as strength, modulus of elasticity, heat resistance, and flame retardancy among organic fibers, they are applied for various uses owing to these characteristics. However, in an application where the heat resistance and flame retardancy are particularly required, because of high strength and high modulus of elasticity, the fibers are not easy to be cut, inferior in post-processability, and thus desired to have improved post-processability.
As a method for improving the post-processability, a method of considerably decreasing the strength of the fibers is supposed to be possible. As the method of considerably decreasing the strength of polybenzazole fiber and pyridobisimidazole fiber, a method of decreasing the concentrations or molecular weights of the polymer, or a method of heating the fiber at a high temperature for a long time can be considered. However, if the concentrations and molecular weights of the polymer are decreased, problems of worsening an operation property may occur; that is, yarn break tends to be caused easily at the time of spinning, switching loss is caused between common brand production and their production, and a spinning conditions also have to be altered because viscosities of dopes are fluctuated significantly. On the other hand, to heat the fiber at a high temperature for a long time, a high temperature furnace is required and a large quantity of energy is required and thus it is also a problem.
While keeping the excellent heat resistance and flame retardancy of the polybenzazole fiber and pyridobisimidazole fiber, a polybenzazole fiber and pyridobisimidazole fiber with rather much lowered strength are desired and it is desired to develop a polybenzazole fiber and pyridobisimidazole fiber excellent in the post-processability while suppressing occurrence of switching loss as much as possible and causing no problem of worsening the operation property without requiring considerable alteration of the process conditions.